Another difference is warranty (most T61p models still come with the IBM style warranty, 3 year onsite service -- which is great). Most of the T61 models do not (they have 1 year carry-in warranty). That might be the biggest difference between your choices (or the specific models you are looking at might have identical warranties). Lenovo (and most other manufacturers) have really made their product lines confusing. While T61p is a higher end model, there are lots of T61s that are higher than some T61p models!!!
Based on your comments here and the "similar" pricing, the T61p is your best choice (no contest). It has a fabulous screen -- a lot better than most laptops -- so the small text size is fine. Also current Windows (and most apps such as Office) make it easy to increase the font size; dpi was an issue when the font size was not selectable in windows, and got smaller as the dpi increased (with no way to bring it back to the original size). Now high dpi displays are a *good* thing, and not to be avoided (as long as you have Vista or XP).

Be sure to get 64 bit Windows if you do get 4GB (32 bit windows uses 2GB max by default, up to 3.5GB by special tweaking, and can never use 4GB). More important than >2GB for most people (and for the app load you described) is getting 7200rpm hard drive. Vista with 2GB and 7200 rpm drive is generally faster than with 4GB and 5400 rpm drive, for most workloads, fwiw. Lenovo also offers the cool 1GB Readyboost flash on the motherboard option, which works great (but some report problems with this new technology). It works a lot better than using USB flash memory (speeds boot by a lot, over 50% faster).